Angular position measuring devices, also frequently referred to as rotary encoders, are used to measure rotational movements of a rotationally mounted machine part, particularly a shaft, over one or more revolutions. In so doing, the rotational movement is recorded incrementally or absolutely. In connection with gear racks and gear wheels or with threaded spindles, linear motions can also be measured by an angular position measuring device. Such angular position measuring devices are often used in conjunction with electric motors to generate a commutation signal.
As a rule, angular position measuring devices of this type have an assembly which includes a shaft having a measuring standard, and a roller bearing. Angular position measuring devices which must satisfy comparatively high demands with regard to measuring accuracy frequently operate according to an optical measuring principle, the measuring standard then often being arranged as a glass ring, for example, having an angle scaling applied thereon. In this case, the angle scaling is scanned by an incident light technique or by a transmitted light technique. In order to ensure high measuring accuracy, it is important that the measuring standard be protected from contaminants. It may be particularly disadvantageous if lubricant or components thereof escape from the roller bearing and deposit on the measuring standard, especially in the area of the angle scaling. Such contaminants are often in the form of droplets, and exhibit an optical effect, similar to a lens. Under such circumstances, the light beams are deflected accordingly, which can lead to a faulty measurement.
Japanese Patent Document No. 2001-289668 describes an assembly for an angular position measuring device which has a shaft on which a separate add-on member is mounted. A measuring standard is affixed on the add-on member. A channel is provided between the add-on member and the shaft to drain off lubricant.
An assembly of this kind has the disadvantage that it is generally accompanied by comparatively great measuring errors. The reason is that on one hand, there is the danger that the measuring standard will not be centered with sufficient precision relative to the axis of rotation, and in addition, that wobble errors will occur. Furthermore, in a conventional arrangement, lubricant can escape from the channel again, or fluids from the interior of the hollow shaft can get onto the measuring standard via the channel.